How to read files in linux kernel after 5.10+ [duplicate] - c

Question

I know all the discussions about why one should not read/write files from kernel, instead how to use /proc or netlink to do that. I want to read/write anyway. I have also read
Driving Me Nuts - Things You Never Should Do in the Kernel.
However, the problem is that 2.6.30 does not export sys_read(). Rather it's wrapped in SYSCALL_DEFINE3. So if I use it in my module, I get the following warnings:
WARNING: "sys_read" [xxx.ko] undefined!
WARNING: "sys_open" [xxx.ko] undefined!
Obviously insmod cannot load the module because linking does not happen correctly.
Questions:
How to read/write within kernel after 2.6.22 (where sys_read()/sys_open() are not exported)?
In general, how to use system calls wrapped in macro SYSCALL_DEFINEn() from within the kernel?

Answer 1

You should be aware that you should avoid file I/O from within Linux kernel when possible. The main idea is to go "one level deeper" and call VFS level functions instead of the syscall handler directly:
Includes:
#include <linux/fs.h>
#include <asm/segment.h>
#include <asm/uaccess.h>
#include <linux/buffer_head.h>
Opening a file (similar to open):
struct file *file_open(const char *path, int flags, int rights)
{
struct file *filp = NULL;
mm_segment_t oldfs;
int err = 0;
oldfs = get_fs();
set_fs(get_ds());
filp = filp_open(path, flags, rights);
set_fs(oldfs);
if (IS_ERR(filp)) {
err = PTR_ERR(filp);
return NULL;
}
return filp;
}
Close a file (similar to close):
void file_close(struct file *file)
{
filp_close(file, NULL);
}
Reading data from a file (similar to pread):
int file_read(struct file *file, unsigned long long offset, unsigned char *data, unsigned int size)
{
mm_segment_t oldfs;
int ret;
oldfs = get_fs();
set_fs(get_ds());
ret = vfs_read(file, data, size, &offset);
set_fs(oldfs);
return ret;
}
Writing data to a file (similar to pwrite):
int file_write(struct file *file, unsigned long long offset, unsigned char *data, unsigned int size)
{
mm_segment_t oldfs;
int ret;
oldfs = get_fs();
set_fs(get_ds());
ret = vfs_write(file, data, size, &offset);
set_fs(oldfs);
return ret;
}
Syncing changes a file (similar to fsync):
int file_sync(struct file *file)
{
vfs_fsync(file, 0);
return 0;
}
[Edit] Originally, I proposed using file_fsync, which is gone in newer kernel versions. Thanks to the poor guy suggesting the change, but whose change was rejected. The edit was rejected before I could review it.

Answer 2

Since version 4.14 of Linux kernel, vfs_read and vfs_write functions are no longer exported for use in modules. Instead, functions exclusively for kernel's file access are provided:
# Read the file from the kernel space.
ssize_t kernel_read(struct file *file, void *buf, size_t count, loff_t *pos);
# Write the file from the kernel space.
ssize_t kernel_write(struct file *file, const void *buf, size_t count,
loff_t *pos);
Also, filp_open no longer accepts user-space string, so it can be used for kernel access directly (without dance with set_fs).